Treatment of rubber



2,710,885 Patented June 14, 1955 Application December 29, 1952, Serial No. 328,516

Claims priority, application France May 16, 1950 2 Claims. ((31. 26082l) No Drawing.

This invention relates to a new process of sensitization to heat of alkaline Hevea latex by means of the zinc salt I grams of solid rubber, 2

of mercapto benzimidazole, and is a continuation-in-part to our application No. 324,658 filed May 4, 1951, now abandoned.

The zinc salt of mercaptobenzimidazole is referred to on pages 436 to 445 of volume 24, No. 12, of the Revue Gnrale du Caoutchouc, published by R. Hildenrand.

It is known that moulded objects or dipped articles can be obtained by heat coagulation of previously sensitized latices, but the known processes have a certain number of disadvantages which are set out below.

The use of ammonium salts entails washing after coagulation. Organic products, such as the diarylguanidines, avoid the necessity for this washing, but they are less effective, modify the conditions of vulcanisation of the mixture and often exercise an unfavourable action on the keeping properties of the articles. Zinc oxide only sufficiently sensitizes latices which are degraded, or at least only slightly stabilized. The use of a ferment allows, by biochemical degradation, of bringing all the ammoniated latices to a state suitable for thermosensitization by zinc oxide in the presence of vulcanisation agents. In practice, however, it is rather diflicult to obtain uniform results, the biological processes being extremely complex and capable of being influenced in their development by numerous factors, such as the original state of the latex, variations in temperature during the course of the digestion by the ferment, etc. The use of polyvinyl others, which hasbeen studied more specially for the latices of synthetic rubber, is only applicable to latices which are weakly stabilized and in any event the polyvinyl ethers are rather unstable in character. The use of electrolytes entails Washing after coagulation and requires, for the manufacture of moulded objects, latices which are strongly stabilized by the addition of protective colloids. Finally, although mercaptobenzimidazole has been proposed it has not been used to any great'extent on account of difiiculties in its application. Mixtures thermosensitized by mercaptobenzimidazole in fact lose their sensitivity to heat in the course of storage.

It has been found that alkaline Hevea latices containing formaldehyde, ammonia, hexamethylene tetramine, sodium sulphite, sodium pentachlorophenate, or less than 10 grams per litre of a soap or mixtures thereof as a preservative rendered heat sensitive according to the present invention, that is to say, by the addition of the zinc salt of mercaptobenzimidazole coagulate on heating under the same conditions immediately after their preparation or after a prolonged rest of several days.

The alkaline Hevea latices of this invention may or may not be buffered by means of their natural amino acids or by additional amino acids, and they comprise in practice:

(a) Weakly and temporarily preserved latices,

(b) Ammoniated latices whether creamed or centrifuged and (c) Mixtures of the latices designated under (a) and The latices included in the invention are rendered thermosensitive by the addition of 0.5% to 5% of the zinc salt of mercaptobenzimidazole (based on the weight of solid rubber contained in the latex); they can contain in addition vulcanising agents, anti-oxidants, dyestuffs, mineral fillers, dispersing agents for mineral fillers, wetting agents or stabilisers.

As it is quite evident that a choice of the respective proportions of the stabilising agents and fillers can modify the stability in the cold of the mixtures with a base of latex and, as a result, their behaviour when thermosensitized there is given hereinafter a test enabling one to recognise whether a mixture with a base of alkaline Hevea latex comes within the scope of the invention: Introduce into a quantity of the mixture to be tested containing 100 grams of the zinc salt of mercaptobenzimidazole in the form of a 33% aqueous dispersion which has. been prepared by grinding a mixture of the following formula for 90 hours on a ball-grinding mill- 33 parts of the zinc salt of mercaptobenzimidazole, 2.5 parts of dispersing agent (sodium salt of the condensation product of formaldehyde with naphthalene-2-sulphonic acid), 5 parts of aseptic aqueous solution 0.4% of gum tragacanth, and 59.5 parts of distilled water.

-, The mixture falls, within the invention if, after thermosensitization as given above, it satisfies the two following conditions which are not satisfied in the absence of the zinc salt of mercaptobenzimidazole.

(1) It is neither coagulated nor gelled after standing for 1 hour at 20 C.

(2) It is coagulated or gelled in less than 1 hour at 90 0., if 100 ccms. of the mixture are placed in a tube having a capacity of at most 0.25 1., corked and plunged into a water bath.

The process of thermosensitization thus described has value as a test but it is well understood that the mixtures with a base of alkaline Hevea latex satisfying this test are usable in the scope of this invention by regulating the amount of zinc salt of mercaptobenzimidazole to between 0.5% and 5% according to Whether it is desired to obtain a more or less rapid gelification or a coagulation and at temperatures between 60 C. and 100 0., these values being understood as those temperatures which can be attained by the mixture and not those of the surrounding media which can be different according to the heat capacity of these media.

The information reproduced in the comparative table below illustrates the differences of behaviour, from the point of view of thermosensitization, of the zinc salt of mercaptobenzimidazole and mercaptobenzirnidazole.

Time necessary for coagulation at 80 0.

Preparation and nature of coagulant The day of After standing 12 preparation days 1 part of mercaptobenzimidazoleu. N0 coa ulation 2mm. 30 sec 2 parts of mercaptohenzimidazole.. 2 min. 10 sec 1 hour at 2 parts of zinc salt of mercapto- 2 min. 15 sec. 2 min. 45 see.

benzimidazole.

The mixtures used were of the following type.

X parts of mercaptobenzimidazole or the corresponding zinc salt.

The zinc salt of mercaptobenzirnidazole has the further advantage over mercaptobenzimidazole itself that it does not bring about a thickening of the latex which is so troublesome.

Mixtures of the type indicated above have, on the day of their preparation, the following Ford viscosities (taken at 20 C.).

The use of the Zinc salt of mercaptobenzimidazole in stead of mercaptobenzirnidazole for the Sensitization of alkaline Hevea latices to heat is therefore characterised by two new and unforeseeable properties, namely the permanence of sensitization conferred to heat, and the small alteration of the viscosity of the mixtures.

These properties in conjunction with the properties already known of the zinc salt of mcrcaptobenzirnidazole make the process of sensitization to heat, according to the invention, an ideal process combining the following advantages:

(1) Suitable sensitivity to when their stability is increased by the addition of ammonia, soaps and protective colloids.

(2) Remarkable stability to cold of latices sensitizer to heat.

(3) Efiicacy of thermosensitization even in absence of zinc oxide.

(4) Regularity of the conditions of coagulation.

(5) Absence of washing after coagulation.

(6) Compatibility of the zinc salt of mercaptobenzimidazole with all the ingredients usually employed in the rubber and latex industry.

(7) No influence on the conditions of vulcanisation.

(8) Improvement in the keeping of articles, the zinc salt of mercaptobenzimidazole being an anti-oxidant.

(9) No influence the zinc salt of mercaptobenzimidazole not colouring them even after exposure to light.

(10) No influence on the absorption of water, the zinc salt of mercaptobenzimidazole not being hygroscopic.

(l1) Constant characteristics and good keeping on storage of the thermosensitizing agent.

(12) Extended possibilities of use.

The invention relates moreover by way of particular applications tothe preparation of coagulated rubber according to the process of the invention; the manufacture, starting from alkaline Hevea latices thermosensitized according to the process of the invention, of solid or hollow moulded objects, non-porous or porous of cellular, continuously coagulated articles (threads, tubes, cloths,

heat of the latices even on the coloration of the articles,

4 coatings, etc.) and dipped articles with reinforced walls. invention.

Example 1.T/zerm0sensitive mixture for non-porous moulded articles The following non-limitative examples illustrate the 166 parts of alkaline Hevea latex (centrifuged to 3 parts of zinc oxide 2 parts of sulphur l part of ethyl phenyl dithiocarbamate of zinc 2 parts of zinc salt of mercapto benzirnidazole 0.2 part of dispersing agent (Distabex L solution) 0.5 part of titanium oxide 1 part of pigment dyestufi (Latexol blue solid S. D.

cone.)

5 parts of water The mixture coagulates in 5 minutes at C. on a hot water-bath. The article is then removed from the mould, dried and vulcanized in hot air for 30 minutes at 110 C.

Example 2 A mixture identical with that of Example 1 coagulates in 20 minutes at -100 C. in hot air.

Example 3.Thermoseusitive mixture for cellular articles To a mixture identical with that of Example 1 there is added 25 parts of a 5% aqueous solution of Marseilles soap and the whole is vigorously agitated in a planetary mixer until the volume is approximately tripled. The mixture thus prepared coagulates in 10 minutes at 95 C. in hot air.

Example 4.Tl1erm0sensitive latex 166 parts of alkaline Hevea latex (centrifuged at 60%) 2 parts of zinc salt of mercaptobenzimidazole 0.1 part of dispersing agent (Distabex L solution) 4 parts of water The latex thus sensitized coagulates in about 5 minutes at 80 C. on the water-bath.

We claim:

1. A process of thermosensitization of alkaline Hevea latex containing a preservative of the group consisting formaldehyde, ammonia, hexamethylene tetramine, sodium sulphite, sodium pentachlorophenate, less than 10 grams per litre of a soap and mixtures thereof which comprises incorporating therein between 0.5% and 5% of the zinc salt of rnercaptobenzirnidazole based on the weight of solid rubber contained in the latex.

2. A process for the manufacture of non-vulcanised masses of natural rubber comprising the coagulation by heat, at a temperature between 60 C. and C. of alkaline Hevea latex preserved and thermosensitized according to claim 1.

References Cited in the file of this patent UNITED STATES PATENTS 2,342,526 Borton Feb. 22, 1944 

1. A PROCESS OF THERMOSENSITIZATION OF ALKALINE HEVEA LATEX CONTAINING A PRESERVATIVE OF THE GROUP CONSISTING OF FORMALDEHYDE, AMMONIA, HEXAMETHYLENE TETRAMINE, SODIUM SULPHITE, SODIUM PENTACHLOROPHENATE, LESS THAN 10 GRAMS PER LITRE OF A SOAP AND MIXTURES THEREOF WHICH COMPRISES INCORPORATION THEREIN BETWEEN 095% AND 5% OF THE ZINC SALT OF MERCAPTOBENZIMIDAZOLE BASE ON THE WEIGHT OF SOLID RUBBER CONTAINED IN THE LATEX. 